Dietary proteins (Figure 10.3) are necessary to supply the amino acids needed for growth and the general repair and maintenance of tissues. A daily intake of about 65 g and 50 g of dietary protein is required in males and females respectively, which provides about 10–15% of the total energy in a balanced diet, although only about 5% of body energy comes from their catabolism under normal circumstances. Protein must be obtained from a variety of sources to supply all essential amino acids. Humans are unable to synthesize nine of the 20 amino acids found in proteins that have codons in the genetic code and these are therefore essential dietary constituents (Table 10.2).
However, the nonessential amino acids can be synthesized if the supply of the essential ones is adequate. The ‘quality’ of dietary protein is important and protein intake needs to be varied, particularly as some plant proteins lack one or more of the essential amino acids.